(a) Field of the Invention
The present invention relates to liquid crystal displays (referred to as an LCD hereinafter) and manufacturing methods thereof, and more particularly, to electrostatic discharge prevention circuits for LCDs and manufacturing methods thereof.
(b) Description of the Related Art
A liquid crystal display (LCD), which is one type of flat panel display (FPD), includes two substrates having transparent electrodes and a liquid crystal layer interposed between the substrates. In the LCD, light transmittance is controlled by varying the voltages applied to the liquid crystal layer.
On a thin film transistor (TFT) substrate of the LCD, N gate lines and M data lines, cross one another and define a plurality of pixels in an N×M matrix. A pixel electrode is formed for each pixel, and is coupled to the gate and the data lines by a switching device such as the TFT. The TFT controls display signals transmitted through the data line according to the states of the scanning signals transmitted through the gate line.
The majority of the LCD manufacturing process is performed on a glass substrate. Since the glass substrate in nonconductive, electric charges generated suddenly cannot be dispersed. Such an electrostatic charge may damage the insulating films or TFTs.
In the LCD manufacturing process, the high voltage electrostatic charges generated after the assembly of the TFT substrate and a cooler filter substrate may decrease the substrate quality, even though the charge amount is small. Also, the electrostatic charge generated during the cutting step of the substrate flows into the active area of the pixel regions through gate and data pads and damages the channels of the TFTs near the pads.
FIG. 1 shows a layout view of the conventional LCD substrate which is damaged by an electrostatic discharge. As shown in the drawing, the LCD panel includes a TFT substrate 10 and a cooler filter substrate 20. A pad area 30, in which pads are formed to connect each wire of the TFT substrate 10 to driving circuits, and an active area 40, where actual images are displayed, are separately formed of the TFT substrate 10.
Lines 50 in the active area 40 illustrate pixels of TFTs damaged by electrostatic charges. If electrostatic charges are generated in the pad area 30 and propagate into the active area 40, the channels of the TFTs next to the pads are damaged and the channel quality is deteriorated.
The deteriorated TFT is shown in FIG. 2. As shown in the drawing, a gate line 60 and a data line 80 cross each other. An edge of a gate electrode 61, extended from the gate line 60, overlaps an end of a source electrode 81 which is extended from the data line 80. An edge of the gate electrode 61, opposite the edge overlapping with the source electrode 81, overlaps with a drain electrode 82. A semiconductor film 70 is formed on the overlapping portion of the gate electrode 61, source electrode 81 and drain electrode 82.
The electrostatic charges entering into the TFT will be discharged with sparks between the source electrode 81 and the drain electrode 82, thereby damaging the semiconductor film 70.
To reduce the damage caused by electrostatic discharges, a shorting bar is typically used to disperse the electrostatic charges. The shorting bar is located at the edge of the substrate and connects all the metal wires. However, the shorting bar alone cannot prevent damage caused by a large the electrostatic discharge. Moreover, electrostatic charges cannot be prevented from entering into the substrate after the shorting bar is removed.
In manufacturing the LCD panel, a polarizer is attached after performing a visual display test by applying signals to the shorting bar. Then, the mother substrate is cut into individual LCD substrates, liquid crystal is injected between the substrates, and the injection holes are sealed. The shorting bar is removed as the substrate is cut. In another visual display test, different test signals are applied to adjacent data lines via corresponding parts by probes and driving circuits are attached to the LCD panel.
As mentioned above, since the shorting bar is removed as the substrate is cut, it is difficult to protect the substrate against the electrostatic charges subsequent to the removal of the shorting bar. Moreover, since the polarizers are attached after the simple test, in which only one signal is applied to every wire, by using the shorting bar, the polarizers will be evenly attached to the damaged LCD panel. If a panel is determined to be damaged in a subsequent test, it has to be discarded along with the polarizers, thereby increasing overall manufacturing costs of the LCD.